Iranian President Hassan Rohani has blamed Arab separatists for a deadly attack on a military parade and accused an unnamed U.S.-backed Persian Gulf state of supporting them.

Rohani's comments came as Iran's powerful Islamic Revolutionary Guards Corps (IRGC) vowed 'deadly and unforgettable' revenge for the September 22 attack that killed at least 25 people, including 12 of their comrades, and wounded 60 others.

Iranian state media reported that gunmen dressed in military uniforms opened fire at the event in the southwestern city of Ahvaz, targeting a stand where Iranian officials were gathered to watch an annual event marking the start of the country's 1980-88 war with Iraq.

The attack occurred in the province of Khuzestan, which borders Iraq and has a large ethnic Arab community.

The province has seen separatist violence in the past that Iran has blamed on its regional rivals.

Tehran also summoned diplomats from Denmark, the Netherlands, and Britain on September 23 for allegedly hosting members of the group suspected of links to the Ahvaz National Resistance, an antigovernment Arab separatist group who claimed responsibility for the attack.

The Islamic State (IS) extremist group also claimed responsibility.

'It is absolutely clear to us who has done this, which group it is and to whom they are affiliated,' Rohani said on state television on September 23, shortly before leaving Tehran for the UN General Assembly in New York.

'One of the countries in the south of the Persian Gulf took care of their financial, weaponry, and political needs,' he added.

Rohani's remarks could refer to Saudi Arabia, the United Arab Emirates (U.A.E.), or Bahrain, all close U.S. military allies that view Iran as a regional threat because of its support for militant groups across the Middle East.

'All of those small mercenary countries that we see in this region are backed by America,' Rohani said.

WATCH: Gunmen Attack Military Parade In Iran

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The United States and United Nations have condemned the attack, one of the deadliest in the Islamic republic in recent years.

But U.S. Ambassador to the UN Nikki Haley responded to Tehran's accusations of U.S. involvement by saying the attack happened because Rohani had 'oppressed his people for a long time'.

'He needs to look at his own base to figure out where that's coming from. I think the Iranian people have had enough,' Haley said on September 23.

Meanwhile, the IRGC said in a statement carried by state media that the culprits would 'face a deadly and unforgettable vengeance in the near future.'

London-based opposition channel Iran International TV aired an interview on September 22 with Yaqoub Hor Altostari, presented as a spokesman for the Ahvaz National Resistance, who indirectly claimed responsibility for the attack and called it 'resistance against legitimate targets'.

Danish Foreign Minister Anders Samuelsen condemned the attack and stressed that there would be 'consequences' if it turned out that those responsible have connections to Denmark.

Iran later summoned the U.A.E.'s envoy as well over what it called the 'irresponsible and insulting statements' of an U.A.E. adviser, according to the semiofficial ISNA news agency. The U.A.E. did not immediately acknowledge the summons.